Fishery Data Series No. 03-16

Abundances, Length and Age Compositions, and CPUE of Northern Pike Within Selected Sloughs of the Nowitna River, 1997

by John Burr and Stafford Roach

August 2003 Department of Fish and Game Division of Sport Fish Symbols and Abbreviations The following symbols and abbreviations, and others approved for the Système International d'Unités (SI), are used in Division of Sport Fish Fishery Manuscripts, Fishery Data Series Reports, Fishery Management Reports, and Special Publications without definition.

Weights and measures (metric) General Mathematics, statistics, fisheries

centimeter cm All commonly accepted e.g., Mr., Mrs., alternate hypothesis HA deciliter dL abbreviations. a.m., p.m., etc. base of natural e gram g All commonly accepted e.g., Dr., Ph.D., logarithm professional titles. R.N., etc. hectare ha catch per unit effort CPUE and & kilogram kg coefficient of variation CV at @ χ2 kilometer km common test statistics F, t, , etc. Compass directions: liter L confidence interval C.I. east E meter m correlation coefficient R (multiple) north N metric ton mt correlation coefficient r (simple) south S milliliter ml covariance cov west W millimeter mm degree (angular or ° Copyright  temperature) Weights and measures (English) Corporate suffixes: degrees of freedom df cubic feet per second ft3/s Company Co. divided by ÷ or / (in foot ft Corporation Corp. equations) gallon gal Incorporated Inc. equals = inch in Limited Ltd. expected value E mile mi et alii (and other et al. fork length FL ounce oz people) greater than > ≥ pound lb et cetera (and so forth) etc. greater than or equal to quart qt exempli gratia (for e.g., harvest per unit effort HPUE example) yard yd less than < id est (that is) i.e., ≤ less than or equal to latitude or longitude lat. or long. logarithm (natural) ln monetary symbols $, ¢ Time and temperature logarithm (base 10) log (U.S.) day d logarithm (specify base) log2, etc. months (tables and Jan,...,Dec mideye to tail fork MEF degrees Celsius °C figures): first three degrees Fahrenheit °F letters minute (angular) ' hour (spell out for 24-hour clock) h number (before a # (e.g., #10) multiplied by x minute min number) not significant NS second s pounds (after a number) # (e.g., 10#) null hypothesis HO registered trademark  percent % trademark  probability P Physics and chemistry U.S. probability of a type I α all atomic symbols (adjective) error (rejection of the null hypothesis when alternating current AC United States of USA America (noun) true) ampere A β U.S. state and District use two-letter probability of a type II calorie cal of Columbia abbreviations error (acceptance of direct current DC abbreviations (e.g., AK, DC) the null hypothesis when false) hertz Hz horsepower hp second (angular) " hydrogen ion activity pH standard deviation SD parts per million ppm standard error SE parts per thousand ppt, ‰ standard length SL volts V total length TL watts W variance var

FISHERY DATA SERIES NO. 03-16

ABUNDANCES, LENGTH AND AGE COMPOSITIONS, AND CPUE OF NORTHERN PIKE WITHIN SELECETED SLOUGHS OF THE NOWITNA RIVER, 1997

by

John Burr and Stafford Roach Division of Sport Fish, Fairbanks

Alaska Department of Fish and Game Division of Sport Fish, Research and Technical Services 333 Raspberry Road, Anchorage, Alaska 99518-1599

August 2003

Development and publication of this manuscript were partially financed by the Federal Aid in Sport Fish Restoration Act (16 U.S.C. 777-777K) under Project F-10-13, Job Nos. R-3-4(d) and R-3-4(e).

The Fishery Management Reports series was established in 1989 for the publication of an overview of Division of Sport Fish management activities and goals in a specific geographic area. Fishery Management Reports are intended for fishery and other technical professionals, as well as lay persons. Fishery Management Reports are available through the Alaska State Library and on the Internet: http://www.sf.adfg.state.ak.us/statewide/divreports/html/intersearch.cfm This publication has undergone regional peer review.

John Burr and Stafford Roach Alaska Department of Fish and Game, Division of Sport Fish, Region III, 1300 College Road, Fairbanks, AK 99701-1599,USA

This document should be cited as: Burr, J. and S. M. Roach. 2003. Abundances, length and age compositions, and CPUE of northern pike within selected sloughs of the Nowitna River, 1997. Alaska Department of Fish and Game, Fishery Data Series No. 03-16, Anchorage.

The Alaska Department of Fish and Game administers all programs and activities free from discrimination based on race, color, national origin, age, sex, religion, marital status, pregnancy, parenthood, or disability. The department administers all programs and activities in compliance with Title VI of the Civil Rights Act of 1964, Section 504 of the Rehabilitation Act of 1973, Title II of the Americans with Disabilities Act of 1990, the Age Discrimination Act of 1975, and Title IX of the Education Amendments of 1972. If you believe you have been discriminated against in any program, activity, or facility, or if you desire further information please write to ADF&G, P.O. Box 25526, Juneau, AK 99802-5526; U.S. Fish and Wildlife Service, 4040 N. Fairfield Drive, Suite 300, Arlington, VA 22203 or O.E.O., U.S. Department of the Interior, Washington DC 20240. For information on alternative formats for this and other department publications, please contact the department ADA Coordinator at (voice) 907-465-4120, (TDD) 907-465-3646, or (FAX) 907-465-2440.

TABLE OF CONTENTS Page LIST OF TABLES ...... ii

LIST OF FIGURES...... ii

LIST OF APPENDICES ...... ii

ABSTRACT ...... 1

INTRODUCTION...... 1 Research Objectives ...... 1 Description of the Nowitna River Study Area...... 2 Description of the Nowitna River Northern Pike Fishery ...... 2 METHODS...... 2 Sampling Techniques ...... 4 Estimation of Abundance ...... 4 Estimation of Length and Age Composition ...... 6 Estimation of CPUE ...... 6 RESULTS...... 6

DISCUSSION ...... 15

ACKNOWLEDGMENTS...... 16

LITERATURE CITED...... 16 APPENDIX A: Data File Listing ...... 19 APPENDIX B: Record of Tag Colors and Tag Numbers ...... 21

i LIST OF TABLES Table Page 1. Fin clips used as secondary marks during the marking and recapture events by month...... 4 2. Number of northern pike ≥ 550 mm FL marked (M), captured and examined for marks (C), recaptured with marks (R), and recapture rates (R/C) by study area and month...... 7 3. Estimated abundances and 95% confidence intervals of northern pike ≥ 550 mm FL by study area and month...... 8 4. Minimum, maximum, average, and standard deviation of northern pike lengths sampled by study area and month...... 10

LIST OF FIGURES Figure Page 1. Nowitna River study area...... 3 2. Estimated densities (northern pike per hectare) and 95% confidence intervals of northern pike ≥ 550 mm FL by study area and month...... 9 3. Proportions by length class of northern pike sampled by study area and month ...... 11 4. Proportions by age class of northern pike sampled by study area and month...... 12 5. Cumulative length distributions of northern pike sampled by study area, month, and gear ...... 13 6. CPUE of northern pike by study area, month, and gear...... 14

LIST OF APPENDICES Appendix Page A1. Data files used to estimate parameters of Nowitna River northern pike, 1997...... 20 B1. Floy tag numbers used for Nowitna River northern pike mark-recapture experiments by year and color ...22 B2. Sample sizes by length category for Nowitna River northern pike, 1997...... 23 B3. Sample sizes by age for Nowitna River northern pike, 1997...... 24 B4. Location of release and recapture of northern pike in the Nowitna River within and between sampling periods in 1997...... 25

ii ABSTRACT Abundances, compositions, and catch per unit effort (CPUE) of northern pike Esox lucius within Johnson, Titus, and Pat Moore sloughs of the Nowitna River were described using mark-recapture techniques. Variability in abundance and composition of northern pike among these small areas were examined to determine if future population assessment could be simplified by sampling small areas with a minimum of effort as an index of the larger area. Estimated abundance and density of northern pike varied by slough and month. Estimated abundances ranged from 20 fish (SE = 5) in Pat Moore Slough in August to 1,247 fish (SE = 375) in Titus Slough in August. Estimated densities ranged from 1.6 fish per hectare (SE = 0.4) in Pat Moore Slough in August to 39.9 fish per hectare (SE = 15.1) in Johnson Slough in June. The average length of northern pike sampled ranged from 606 mm FL (SD = 140) in July from Johnson Slough to 744 mm FL (SD = 148) in June from Pat Moore Slough. The minimum age sampled was age-2 and the maximum age-15. There were significant differences between length distributions of northern pike sampled by slough (Takn = 4.85; P < 0.01), month (Takn = 4.55; P < 0.01), and gear (Takn = 2.09; P = 0.04). CPUE with gill nets ranged from 2.51 northern pike per hour in June from Titus Slough to 0.09 per hour in July from Pat Moore Slough. CPUE with hoop traps ranged from 0.28 northern pike per hour in June from Titus Slough to 0.02 per hour in July from Titus Slough. CPUE with hook-and-line ranged from 2.3 northern pike per hour in June from Johnson Slough to 0.0 per hour in August from Titus Slough. There was no obvious correlation between CPUE and abundance. This study does not support the hypothesis that investigators can take a quick look at a small area within the lower Nowitna River to assess the health of the northern pike population in general. Key Words: Northern pike, Esox lucius, population abundance, age composition, length composition, Nowitna River, , mark-recapture, CPUE INTRODUCTION The Alaska Department of Fish and Game (ADF&G) initiated northern pike Esox lucius studies in the Arctic-Yukon-Kuskokwim Region of Alaska (AYK) to ensure that annual harvests do not exceed surplus production of northern pike. Objectives designed to meet this goal have included estimates of abundance, length composition, age composition, mortality rates, recruitment, and movements of northern pike within selected lakes and wetland complexes in AYK. These efforts have focused upon the more accessible fisheries. The Nowitna River, while remote, was recently identified for study due to the increased effort of guided and unguided anglers. The goal of this study was to obtain information on the current stock of northern pike inhabiting the lower Nowitna River. Mark-recapture experiments were conducted within three sloughs at three times during the summer of 1997 to examine variability in abundance and composition among selected locations within the lower river. The hypothesis was that if there were little variability, future assessment could be simplified by examining small areas, with a minimum of effort, as an index of the larger area. RESEARCH OBJECTIVES Abundances, compositions, catches per unit effort of northern pike within selected sloughs of the Nowitna River were determined in 1997. The research objectives were to: 1) estimate the abundance of northern pike ≥ 400 mm FL inhabiting Johnson, Titus, and Pat Moore sloughs during sample periods in June, July, and August such that the estimates are within 25% of the actual value 90% of the time; 2) estimate length and age compositions of northern pike inhabiting Johnson, Titus, and Pat Moore sloughs during sample periods in June, July, and August such that the estimates are within 10 percentage points of the actual value 95% of the time; and,

1 3) estimate catch per unit effort (CPUE) of northern pike with three gear types: hoop nets, gill nets, and hook-and-line. DESCRIPTION OF THE NOWITNA RIVER STUDY AREA The Nowitna River, a Yukon River tributary, is located approximately 130 km downstream from the mouth of the Tanana River. This nationally designated Wild River flows north into the Yukon River and forms a meandering flood plain that varies in width from 1 to 10 km. The lower 80 km of the river is characterized by a mud-sand bottom and variable water conditions by season, which are influenced by breakup conditions both from the Nowitna drainage and from flooding and backup from the Yukon River. In early summer water velocities exceed 1.5 m/s, and water depths generally range from 6 to 10 m with maximum depths of 18 m or more. In late summer water velocities reduce to 1 – 1.5 m/s, stream widths to 60 – 140 m, and water depths to 1 – 8 m. A detailed description of the Nowitna River morphology and a listing of the fish fauna are provided in USFWS (1991). The study area was located within the lower 30 km of the Nowitna River and consisted of three old river channels (oxbows or sloughs) that are still connected to the main river. These sloughs were easily blocked off to ensure a closed population during each experiment. Extremely variable water levels and flows in the main river precluded effective sampling in the mainstream. DESCRIPTION OF THE NOWITNA RIVER NORTHERN PIKE FISHERY During the most recent five-year period (1993-1997), estimated angler effort has averaged approximately 1,000 angler-days by about 300 guided and unguided anglers (Mills 1994, Howe et al. 1995 - 1998). The number of angling guides officially operating in the refuge has increased from zero in 1992 to six in 1997 (J. Goode, Koyukuk/Nowitna Refuge, Galena, personal communication). Sport fishing occurs throughout the open water season, however, anecdotal reports indicate that the majority of the fishing effort and harvest occurs during September concurrent with hunting activities and within the lower 30 km of the river. Catch and release fishing for northern pike appears to be an important component of the Nowitna River northern pike fishery. Between 1993 and 1997, the estimated harvest of northern pike averaged about 200 fish while estimated northern pike catch averaged about 2,600 fish (Mills 1994, Howe et al. 1995 - 1998). In contrast, during 1991, an estimated 1,600 northern pike were harvested from an estimated catch of 2,700 (M. Mills, ADF&G, Anchorage, personal communication). Between 1993 and 1997, excluding the Tanana River drainage, the Nowitna River drainage accounted for an average of 18% of the estimated catch and an average of 10% of the harvest of northern pike in the Yukon River drainage (Mills 1994, Howe et al. 1995 - 1998). METHODS The 1997 Nowitna River study was designed to estimate abundances, length and age compositions, and CPUE of northern pike within three sloughs of the Nowitna River at three different times through the summer (May 29 – June 9; July 18 – 27; and August 21 - 28). The three sloughs selected for this study were Johnson Slough, Titus Slough (up river of Johnson Slough), and Pat Moore Slough (down river of Johnson Slough; Figure 1). Six-panel multi- filament gill nets (length 45 m, depth 2 m; panels alternating 2.5 cm, 3.8 cm, and 5 cm mesh bar- size), seven-hoop nylon hoop traps (1-m diameter mouth, 4.5-m length), with variable length 2- m

2 Yukon River Pat Moore Slough

Nowitna River

0 1 2 Miles Johnson Slough

Titus Slough

Figure 1.—Nowitna River study area.

3 Table 1.—Fin clips used as secondary marks during the marking and recapture events by month. Event Month Marking Event Recapture Event June Lower Caudal Upper Caudal July Left Pelvic Right Pelvic August Left Pectoral Right Pectoral

deep wings, used as funnels and for blocking), and standard hook and line gear were used to sample northern pike. SAMPLING TECHNIQUES The mouths of each slough were blocked with winged hoop-traps such that fish moving in and out of the slough were determined. These blocking nets were checked at least twice daily. All northern pike were sampled as described below. Within each slough gill nets were deployed evenly and as uniformly as possible by moving nets to new sites within the area throughout the day. All healthy northern pike were released immediately after data collection away from the capture sites. Northern pike were sampled during the marking event. Fish were measured to the nearest 1 mm FL. A minimum of two scales were taken from the preferred zone adjacent to the lateral line above the pelvic fins as described by Williams (1955). Both the left and right side of the dorsal fin were examined for the presence of a Floy tag. If a Floy tag was present, the color and number of the tag recorded; or if not present, a new Floy FD-68 internal anchor tag inserted at the left base of the dorsal fin and the number recorded. A fin of all newly tagged fish was slightly clipped as a secondary mark (Table 1). Northern pike killed during the sampling procedure were not tagged but all other data recorded and the fate (K) clearly noted. Fish leaving or entering the sloughs were clearly identified. During the recapture event, the same procedures were used with the addition that fins were examined closely for recent clips, and a fin slightly clipped to signify capture during the event (Table 1). Tag loss (TL) was clearly noted for northern pike without a Floy tag but with a recent tag wound or recent fin clip. Number of fish captured and length of time gear was fished were recorded. Scales collected in the field on gummed cards were impressed on 20 mil acetate sheets using a Carver press at 241,315 kPa (35,000 psi) heated to 150°C for 150-s. Ages were determined from impressions using a Micron 770 microfiche reader (32X) according to criteria established by Williams (1955), and Casselman (1967). All data files used to estimate parameters of the Nowitna River northern pike population are listed in Appendix A1. Floy tag numbers used for this experiment are listed in Appendix B1. ESTIMATION OF ABUNDANCE Abundances were estimated using a Petersen mark-recapture experiment (Seber 1982) based on the following assumptions: 1) the population was closed;

4 2) all northern pike had the same probability of capture during the marking event or the same probability of capture during the recapture event or marked and unmarked northern pike mixed completely between events; 3) marking northern pike did not affect their subsequent probability of capture; 4) northern pike did not lose their mark between events; and, 5) all marked northern pike were reported when recovered. Blocking off the mouths of each slough during the time of the experiment ensured the validity of assumption 1. Individual northern pike that either exited or entered the study slough during the marking period of the experiments were excluded from the calculation of abundance. Mortality and growth, which may contribute to the violation of assumption 1, were assumed negligible because of the short duration of each experiment. The validity of assumptions 2 and 3 was inferred by uniformly sampling each slough. Chi-square tests to compare catchability (probability of capture) among areas within each slough during the recapture events (the frequency of fish with marks to the frequency of fish without marks) were not performed because of the low number of recaptures. Capture probabilities, however, were assumed similar due to the small area of each of the sloughs. Kolmogorov-Smirnov two-sample tests to determine bias in estimated abundances due to size selectivity were also not performed because of the low number of recaptures. The validity of assumption 4 was ensured by double marking (Floy tag and fin-clip) each northern pike. Tag loss was noted when a fish was recovered with the specific fin clip but without a Floy tag. The validity of assumption 5 was ensured by a thorough examination of fins for fin-clips and the recording of fin clips and Floy tag numbers for all captured northern pike. Estimated abundances of northern pike were calculated from the number of northern pike marked, examined for marks, and recaptured. The Chapman estimator (Seber 1982) was used: ()M+1(C+1) N$ = −1 (1) R1+ where: M = the number of northern pike marked and released alive during the marking event; C = the number of northern pike examined for marks during the recapture event; R = the number of northern pike recaptured during the recapture event; and, N$ = estimated abundance of northern pike at the time of marking.

Variances of the abundance estimates (Seber 1982) were estimated as: ()MM++1 (C1)(−R)(C−R) VN$ []$ = . (2) ()R1++2(R2)

5 ESTIMATION OF LENGTH AND AGE COMPOSITION Length proportions of northern pike captured were estimated for the selected sloughs of the Nowitna River. The proportion and variance estimators used were: x = k p$ k ,and (3) n ()− = pp$$k1 k V$ []p$ k (4) n −1 where: p$ k = the proportion of northern pike that were length group k;

xk = the number of northern pike sampled that were length group k; and, n = the number of northern pike sampled that were measured.

Age composition was estimated using the same equations for proportions and variances of proportions used to estimate length composition except ages were substituted for lengths. Length and age composition data were archived (Appendices B2, B3). The length distributions of northern pike caught by gear, month, and slough were compared using the Anderson-Darling k-sample test (Scholz and Stephens 1987). ESTIMATION OF CPUE CPUE was calculated by slough and month for northern pike captured by hoop traps, gill nets, and hook and line as the number of fish captured divided by the total time the gear was fished. RESULTS There were no recaptured fish < 550 mm FL, therefore, only fish ≥ 550 mm FL were used in estimating abundances of northern pike in Johnson, Titus, and Pat Moore sloughs. Of 752 unique northern pike ≥ 550 mm FL handled during the mark-recapture experiments, 353 were tagged and released alive during the marking events and 451 were examined for marks during the recapture events of which 52 were recaptures from the marking events (Table 2). During the experiments, water temperatures varied from as low as 14° C in June, as high as 24° in July, and as low as 15° in August. During the experiment, 52 northern pike ≥ 550 mm FL (< 7% of fish handled) were inadvertently killed. Twenty-three of these were killed during the recapture event during June in Titus Slough due to lengthy soak times in warm water. Soak times of the gear were reduced accordingly for the remainder of the experiments. Estimated abundances of northern pike within the selected sloughs of the Nowitna River were germane to fish ≥ 550 mm FL. Recapture rates (fish recaptured divided by fish examined for marks in the recapture event; R/C) ranged from 0.00 to 0.50 (Table 2). Bias from unequal catchability by length, gear, or area within a slough could not be determined satisfactorily due to the low number of recaptures in any one slough. By default, abundance was estimated using the

6 Table 2.—Number of northern pike ≥ 550 mm FL marked (M), captured and examined for marks (C), recaptured with marks (R), and recapture rates (R/C) by study area and month. Number of Northern Pike Study Area Month Marked Captured Recaptured R/C Johnson Slough June 26 78 4 0.05 July 39 29 11 0.38 August 34 17 6 0.35

Titus Slough June 52 103 8 0.08 July 83 49 8 0.16 August 91 121 8 0.07

Pat Moore Slough June 7 33 0 0.00 July 11 13 3 0.23 August 10 8 4 0.50 unstratified Chapman estimator based on the hypergeometric probability distribution. Estimated abundances ranged from 20 fish (SE = 5) in Pat Moore Slough in August to 1,247 fish (SE = 375) in Titus Slough in August (Table 3). Estimated densities ranged from 1.6 fish per hectare (SE = 0.4) in Pat Moore Slough in August to 39.9 fish per hectare (SE = 15.1) in Johnson Slough in June (Figure 2). Selectivity by length within any one slough was not reconciled satisfactorily by diagnostic tests because of the low number of recaptures. Therefore, both events were combined and all fish sampled were used to describe the length composition of the catch in each slough. The minimum length of northern pike sampled ranged from 305 mm FL in July from Johnson Slough to 485 mm FL in June from Pat Moore Slough (Table 4; Figure 3). The maximum length of northern pike sampled ranged from 905 mm FL in July from Pat Moore Slough to 1,114 mm FL in July from Johnson Slough (Table 4; Figure 3). The average length of northern pike sampled ranged from 606 mm FL (S.D. = 140) in July from Johnson Slough to 744 mm FL (S.D. = 148) in June from Pat Moore Slough (Table 4; Figure 3). The minimum age sampled was age-2 and the maximum age-16 (Figure 4). There were significant differences between length distributions of northern pike sampled by slough (Takn = 4.85; P < 0.01), month (Takn = 4.55; P < 0.01), and gear (Takn = 2.09; P = 0.04). Samples from Pat Moore Slough contributed the most to the difference by slough, samples in July contributed the most to differences by month, and samples taken with hook and line contributed the most to differences by gear (Figure 5). CPUE with gill nets ranged from 2.51 northern pike per hour in July from Johnson Slough to 0.09 per hour in August from Titus Slough (Figure 6). CPUE with hoop traps ranged from 0.28 northern pike per hour in June from Titus Slough to 0.02 per hour in July from Titus Slough (Figure 6). CPUE with hook and line ranged from 2.30 northern pike per hour in June from Johnson Slough to 0.00 per hour in August from Titus Slough (Figure 6).

7 Table 3.—Estimated abundances and 95% confidence intervals of northern pike ≥ 550 mm FL by study area and month. Number of Northern Pike Study Area Month Nˆ 95% CI Johnson Slough June 427 273 – 1,231 July 100 78 – 152 August 90 64 – 176

Titus Slough June 612 426 – 1,204 July 467 326 – 908 August 1,247 854 – 2,524

Pat Moore Slough June - - July 42 28 – 122 August 20 14 – 38

8 80

Johnson Slough Titus Slough Pat Moore Slough 18 ha 56 ha 12 ha 60 Pike/Hectare

n 40 her t r o N 9 ber 20 um N

0 June July August June July August June July August

Month 1997

Figure 2.—Estimated densities (northern pike per hectare) and 95% confidence intervals of northern pike ≥ 550 mm FL by study area and month.

9

Table 4.—Minimum, maximum, average, and standard deviation of northern pike lengths sampled by study area and month. Length (mm FL) Study Area Month Minimum Maximum Average S.D. Johnson Slough June 440 1,060 699 128 July 305 1,114 606 140 August 407 1,121 671 129

Titus Slough June 330 1,020 661 111 July 401 1,010 668 117 August 325 1,127 681 118

Pat Moore Slough June 485 1,055 744 148 July 455 905 701 99 August 408 961 649 114

10 Johnson Slough

0.60 0.60 0.60 June July August

0.40 0.40 0.40

0.20 0.20 0.20

0.00 0.00 0.00 250 450 650 850 250 450 650 850 250 450 650 850 1,050 1,050 1,050

Titus Slough

0.60 June 0.60 July 0.60 August

0.40 0.40 0.40

0.20 0.20 0.20

0.00 0.00 0.00 Proportions 250 450 650 850 250 450 650 850 250 450 650 850 1,050 1,050 1,050

Pat Moore Slough

0.60 0.60 0.60 June July August

0.40 0.40 0.40

0.20 0.20 0.20

0.00 0.00 0.00 250 450 650 850 250 450 650 850 250 450 650 850 1,050 1,050 1,050 100 mm Fork-Length Classes

Figure 3.—Proportions by length class of northern pike sampled by study area and month. Sample sizes by length category are listed in Appendix B2.

11 Johnson Slough

0.40 0.40 0.40 June July August

0.20 0.20 0.20

0.00 0.00 0.00 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16

Titus Slough

0.40 0.40 0.40 June July August

0.20 0.20 0.20

0.00 0.00 0.00 Proportions 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16

Pat Moore Slough

0.40 0.40 0.40 June July August

0.20 0.20 0.20

0.00 0.00 0.00 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 Age Classes

Figure 4.—Proportions by age class of northern pike sampled by study area and month. Sample sizes by age class are listed in Appendix B3.

12 1.00

0.67 Johnson 0.33 Titus Pat Moore 0.00 200 400 600 800 1,000 1,200

1.00

0.67 June July 0.33 August

0.00 200 400 600 800 1,000 1,200

1.00

0.67 Gill Net Cumulative Distribution Functions Hook & Line 0.33 Hoop Trap

0.00 200 400 600 800 1,000 1,200

Fork Length Figure 5.—Cumulative length distributions of northern pike sampled by study area, month, and gear.

13 Gill Nets 3.00 Johnson Slough Titus Slough Pat Moore Slough

2.00

1.00

0.00 June July August June July August June July August

Hoop Traps

Johnson Slough Titus Slough Pat Moore Slough 0.40

0.20

0.00 June July August June July August June July August

Hook and Line

CPUE (Number/Hour) 3.00 Johnson Slough Titus Slough Pat Moore Slough

2.00

1.00

0.00 June July August June July August June July August

Month 1997 Figure 6.—CPUE of northern pike by study area, month, and gear.

14 DISCUSSION

The paucity of recaptured fish was a nagging problem for the Nowitna River northern pike study. Fish moving into and out of the study sloughs confounded the estimation of abundance by reducing the number of fish available for each experiment. Although careful records were maintained, the consequence of the immigration and emigration was to reduce the number of marked fish available for recapture and hence to reduce the viability of the mark-recapture experiments. Abundances were estimated with as few as three and at most eleven recaptures. Diagnostic tests for equal catchability by length and area were not satisfactory because the low power resultant from few recaptures rendered the tests of little value. Increasing the duration of each experiment or increasing the fishing effort may have improved number of recaptures. The density of northern pike in Johnson Slough was considerably greater in June than in July or August and also greater than in Titus Slough in June. This along with the habitat characteristics of Johnson Slough—shallow water depth and emergent vegetation—suggests that Johnson Slough is a primary spawning area for the lower Nowitna River. This study also indicated that northern pike moved out of Johnson Slough and into Titus Slough—deepest of the three sloughs studied—through the summer as water depth and temperature changed throughout the lower river. Headrick and Carline (1993) reported that northern pike moved to cooler water once the water temperature exceeded 20° C. Furthermore, there is some evidence that smaller fish moved into Johnson Slough after larger spawning-size fish moved out in July or remained in Johnson Slough later than the larger fish. This supports the findings of Neumann and Willis (1995), which showed that several samples taken from a northern pike population from narrow periods of time may indicate differences in length structure when there is no underlying population difference. They suggested that variable activity and habitat use by sex and size in any one location might account for these differences. The data from this study do not support the hypothesis that investigators can take a “snap-shot” look at any one slough during any month of the summer to assess the health of the northern pike population in the lower Nowitna River. Within the same summer, densities and average lengths varied considerably by slough and month. This can be attributed to a preference for spawning area, changing water levels through the summer, and changing water temperatures. Furthermore, there is no reason to believe that these conditions in any one slough would not be variable from one year to the next. In addition, CPUE was not reflective of abundance and provided little information about aggregations of northern pike. This indicated that it is unlikely that a CPUE index from a small area can be used as a surrogate for mark-recapture experiments to assess abundance in a much larger area. Casselman (1978) and Diana (1980) also found that catch rates were not always reflective of population size. This may be attributed to a time specific increase or decrease in northern pike activity at a given location (Roach 1993; 1998a). This study demonstrates some of the problems with attempting to assess the health of a population inhabiting a large area by taking a quick look at a small portion of the total area. By looking at any one site at any single time, investigators can arrive at a myriad of interpretations on the status of the stock. A better method of assessing age and size composition in the Nowitna River northern pike stock would be to select sampling locations throughout the lower 60 to 100 km of the river and to use a

15 combination of gear types to collect the samples. Roach (1998b) found that gear induced size selectivity can be minimized by using a combination of actively fished gill nets and hoop nets. To obtain a more complete assessment of the northern pike stock, a mark recapture experiment would be necessary to better understand biases associated with sampling. The study area and the timing of the experiment must be carefully selected due to the open nature of the river and the potential magnitude of movement of the fish. Radio telemetry has been successfully used in other areas to help design mark/recapture experiments for northern pike in open systems. Roach et al. (1998a) used information from movements of radio tagged pike in Minto Flats to select study areas and time periods which would minimize bias introduced by fish moving into or out of study area(s). A radio telemetry study to describe the timing and magnitude of seasonal movements of Nowitna northern pike is recommended when additional stock status information is required. The Nowitna River continues to be one of the most important destinations for anglers targeting northern pike in the Yukon drainage. Sport fishing regulations are currently very liberal; the daily bag and possession limit for northern pike is ten per day with no size limit. A sustained increase in fishing effort and or harvest will be used as a trigger for assessment of this population. The ADF&G will continue to monitor the statewide harvest survey as a means for detecting changes in use of this fish stock. ACKNOWLEDGMENTS The authors credit and appreciate the help received with the study design and biometric review from Mike Wallendorf during the planning process, fieldwork, and data analysis. Additionally, special thanks go to those who helped collect the data: Lem Butler, John Chythlook, Becky Kelleyhouse, Monica McGilvary, Steve Riener, Tim Viavant, Mike Wallendorf, and Klaus Wuttig. The U. S. Fish and Wildlife Service provided partial funding for this study through the Federal Aid in Sport Fish Restoration Act (16 U.S.C. 777-777K) under project F-10-13, Job Nos. R-3-4-(d) and R-3-4(e). LITERATURE CITED Casselman, J. M. 1967. Age and growth of northern pike, Esox lucius Linnaeus, of the Upper St. Lawrence River. Master’s thesis, University of Guelph. Casselman, J. M. 1978. Effects of environmental factors on growth, survival, activity, and exploitation of northern pike. American Fisheries Society Special Publication 11:114-128. Diana, J. S. 1980. Diel activity pattern and swimming speeds of northern pike (Esox lucius) in Lac Ste. Anne, Alberta. Canadian Journal of Fisheries and Aquatic Sciences 37:1454-1458. Headrick, M. R. and R. F. Carline. 1993. Restricted summer habitat and growth of northern pike in two southern Ohio impoundments. Transactions of the American Fisheries Society 122:228-236. Howe, A. L., G. Fidler, and M. J. Mills. 1995. Harvest, catch, and participation in Alaska sport fisheries during 1994. Alaska Department of Fish and Game, Fishery Data Series Number 95-24, Anchorage. Howe, A. L., G. Fidler, A. E. Bingham, and M. J. Mills. 1996. Harvest, catch, and participation in Alaska sport fisheries during 1995. Alaska Department of Fish and Game, Fishery Data Series Number 96-32, Anchorage. Howe, A. L., G. Fidler, C. Olnes, A. E. Bingham, and M. J. Mills. 1997. Harvest, catch, and participation in Alaska sport fisheries during 1996. Alaska Department of Fish and Game, Fishery Data Series Number 97-29, Anchorage.

16 LITERATURE CITED (Continued) Howe, A. L., G. Fidler, C. Olnes, A. E. Bingham, and M. J. Mills. 1998. Harvest, catch, and participation in Alaska sport fisheries during 1997. Alaska Department of Fish and Game, Fishery Data Series No. 98-25, Anchorage. Mills, M. J. 1994. Harvest, catch, and participation in Alaska sport fisheries during 1993. Alaska Department of Fish and Game, Fishery Data Series Number 94-28, Anchorage. Neumann, R. M. and D. W. Willis. 1995. Seasonal variation in gill-net sample indexes for northern pike collected from a glacial prairie lake. North American Journal of Fisheries Management 15:838-844. Roach, S. M. 1993. Movements and distributions of radio-tagged northern pike in Harding Lake. Alaska Department of Fish and Game, Fishery Data Series Number 93-12, Anchorage. Roach, S. M. 1998a. Site fidelity, dispersal, and movements of radio-implanted northern pike in Minto Lakes, 1995 - 1997. Alaska Department of Fish and Game, Fishery Manuscript Number 98-1, Anchorage. Roach, S. M. 1998b. Abundance and composition of the northern pike population in Minto Lakes, 1997. Alaska Department of Fish and Game, Fishery Data Series Number 98-12, Anchorage. Seber, G. A. F. 1982. The estimation of animal abundance and related parameters. Macmillan, New York. Scholz, F. W., and M. A. Stephens. 1987. K-sample Anderson Darling Tests. Journal of the American Statistical Association 82:918-924. USFWS. 1991. Nowitna fishery management plan. United States Fish and Wildlife Service, Anchorage. Williams, J. E. 1955. Determination of age from the scales of northern pike, (Esox lucius L.). Doctoral Dissertation series Publication Number 12:668, Ann Arbour, Michigan: University Microfilms.

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APPENDIX A DATA FILE LISTING

19 Appendix A1.—Data files used to estimate parameters of Nowitna River northern pike, 1997. Data filea Description

NWT97DAT.XLS Sampling data for northern pike captured during June, 1997.

JULY97.XLS Sampling data for northern pike captured during July, 1997.

AUG97JC.XLS Sampling data for northern pike captured during August, 1997.

a Data files were archived at and are available from the Alaska Department of Fish and Game, Sport Fish Division, Research and Technical Services, 333 Raspberry Road, Anchorage, Alaska 99518-1599.

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APPENDIX B RECORD OF TAG COLORS AND TAG NUMBERS

21 Appendix B1.-Floy tag numbers used for Nowitna River northern pike mark-recapture experiments by year and color. Tag Color Year Green Blue Gray 1997 37,561 - 38,003 64,394 – 64,699 2,375 – 2,625 68,459 – 68,499 18,860 – 18,999 34,600 – 34,977

22 Appendix B2.-Sample sizes by length category for Nowitna River northern pike, 1997. Johnson Slough Titus Slough Pat Moore Slough Length June July August June July August June July August 300 – 349 - 8 - 1 - 3 - - - 350 – 449 1 2 2 4 5 2 - - 1 450 – 549 8 18 7 15 13 16 5 1 - 550 – 649 31 30 20 63 50 72 7 7 13 650 – 749 38 26 16 62 57 67 12 11 1 750 – 849 21 8 11 21 13 45 10 4 3 850 – 949 8 1 2 3 5 12 7 2 - 950 – 1,049 5 1 1 6 7 1 3 - 1

23 > 1,050 1 1 1 - - 2 1 - -

All 208 95 60 175 150 220 45 25 19

Appendix B3.-Sample sizes by age for Nowitna River northern pike, 1997. Johnson Slough Titus Slough Pat Moore Slough Age June July August June July August June July August 1 ------2 - 6 2 1 1 3 - - - 3 - 4 - 4 6 3 - - 2 4 2 4 5 4 7 9 1 - 2 5 14 13 6 14 24 30 5 5 3 6 19 27 20 25 26 17 2 6 - 7 21 13 7 29 26 22 5 5 3 8 13 8 3 13 18 29 6 3 2 24 9 12 1 1 7 10 21 4 3 1 10 4 1 - 2 4 10 2 - - 11 3 1 - - 5 5 - - - 12 2 1 1 1 6 3 2 1 - 13 2 1 1 - 1 1 1 - 14 - 1 - - - - - 15 - 1 1 1 - - - 16 - - - 1 - - -

Appendix B4.-Location of release and recapture of northern pike in the Nowitna River within and between sampling periods in 1997.

June - June June - July Recaptured Recaptured Marked JSNa PMSb TITUSc RIVERd Marked JSN PMS TITUS RIVER JSN 20e 6 JSN 6 PMS 2 PMS TITUS 8 TITUS 4 RIVER 1 RIVER July - July June - August Recaptured Recaptured Marked JSN PMS TITUS RIVER Marked JSN PMS TITUS RIVER JSN 23 5 5 JSN 2 1 1 PMS 2 3 1 PMS 1 TITUS 1 8 TITUS RIVER RIVER August - August July - August Recaptured Recaptured Marked JSN PMS TITUS RIVER Marked JSN PMS TITUS RIVER JSN 8 1 JSN 4 1 PMS 4 PMS 2 TITUS 11 1 TITUS 5 1 RIVER RIVER

a Johnson Slough (mile 11) b Pat Moore Slough (mile 1.5) c Titus Slough (mile 13) d Test net sets in Nowitna River between JSS and Titus (mile 12) e The number of fish marked and recaptured in each slough during each experiment is often greater than the number of recaptures listed in Table 2. The additional numbers reflect fish that moved out of each slough during the marking periods of the experiments.

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